Soon after installing Ubuntu 18.04 I noticed that my WIFI connection was running very slow. After some searching on the internet I found this solution which seems to work for me and should hopefully work for you.
It turns out that there is an issue with the 802.11 protocol and iwlwifi Intel chips. and the solution is to turn it off.
A simple check to see if the solution works
Before making any changes make sure this solution works by running the following command in terminal:
sudo modprobe -r iwlwif
sudo modprobe iwlwifi 11n_disable=1
If this improves the speed then you can make the change permanent with:
echo "options iwlwifi 11n_disable=1" | sudo tee /etc/modprobe.d/iwlwifi.conf
You should now see a visible improvement in network speed.